Peptide growth and differentiation factors elicit responses in target cells by means of specific interactions with receptors at the cell surface. Growth factor receptors are typically membrane glycoproteins with distinct extracellular, transmembrane, and intracellular domains. The structural segregation of the domains corresponds to function (Ullrich et al. Cell 61, 203 (1990)); the extracellular domain appears to be responsible for ligand binding and ligand-mediated receptor dimerization (Cunningham et al. Science 254, 821 (1991); Lev et al. J. Biol. Chem. 267, 10866 (1992)), while the intracellular domain of the receptor, or the intracellular domain of an accessory element (Takeshita et al. Science 257, 379 (1992)), is responsible for signal transduction. Much of the specificity of growth factor activity is dictated by the interaction with the binding site on the extracellular domain of the cognate receptor. Chimeric receptors, engineered to contain the extracellular domain of one receptor and the intracellular domain of a second receptor, retain the ligand specificity of the extracellular component (Lehvaslaiho et al. EMBO J. 8, 159 (1989)). The downstream signaling pathways activated by such chimeric receptors correspond to those activated by the intracellular component. In many cases soluble forms of receptors, consisting of only the extracellular domains, retain ligand binding activity (Lev et al. i; Duan et al. J. Biol. Chem. 266, 413 (1991)). Truncated receptors have been identified in serum (Fernandez-Botran FASEB J. 5, 2567 (1991)), cell culture supernatants (Zabrecky et al. J. Biol. Chem. 266, 1716 (1991)), and have been produced through recombinant techniques (Lev et al. ibid, Duan et al. ibid).
Recent progress in nucleic acid sequencing and amplification technologies has resulted in the identification of an increasing number of genes which code for previously unidentified growth factor receptors (Wilks Proc. Natl. Acad. Sci. USA 36, 1603 (1989); Lai et al. Neuron 6, 691 (1991)). As a result, there is a demand to develop procedures which can define the biological roles of orphan receptors, including techniques which can identify ligands for these receptors (McConnell et al. Science 257, 1906 (1992)) Receptor affinity technology is one approach to this problem. This technology may augment existing strategies for the isolation of novel growth factors, since it allows the detection of ligands when biological responses are subtle or undefined.
Recent reports have suggested that the extracellular domains of receptors can be exploited as growth factor-specific affinity reagents. Bailon et al. (Biotechnology 5, 1195 (1987)) have shown that the extracellular domain of the IL-2 receptor .alpha. subunit can be immobilized on chromatographic media and used for the purification of recombinant IL-2. A genetic fusion of the kit extracellular domain with an alkaline phosphatase enzymatic tag allowed the identification of a cell associated ligand for the receptor (Flanagan et al. Cell 63, 185 (1990)). Lupu et al. (Proc. Natl. Acad. Sci. USA 89, 2287 (1992)) have reported the affinity purification of an activity which binds to the immobilized extracellular domain of the erbB-2 gene product.
The eck gene, originally identified by cDNA cloning from a human epithelial cell library, encodes a 130 kDa receptor-like protein-tyrosine kinase (p130.sup.eck) (Lindberg et al. Mol. Cell. Biol. 10, 6316 (1990)). Immunohistochemical and mRNA screening of tissues and cell lines suggest that eck expression is highest in cells of epithelial origin. By analogy with genes encoding other receptor-like protein-tyrosine kinases, eck may be a proto-oncogene and therefore may have a role in carcinogenesis. This potential role for eck is more likely given the frequent involvement of epithelial cells in human cancers. Receptor protein kinases are typically activated through interaction with one or more ligands. However, a ligand capable of activating p130.sup.eck has not yet been reported. The identification of such a ligand may be important in defining the role of p130.sup.eck activation in the development of some human cancers.
It is therefore an object of this invention to identify one or more ligands for p130.sup.eck. The possible role of p130.sup.eck in the transformation of epithelial cells to a cancerous state suggests that identification of the ligand responsible for receptor activation may have therapeutic implications for some epithelial cell-derived malignancies.